How to Reverse List in Kotlin: Simple Syntax and Examples

In Kotlin, you can reverse a list using the reversed() function which returns a new list with elements in reverse order. To reverse a mutable list in place, use reverse() which modifies the original list.

Syntax

The reversed() function returns a new list with elements reversed without changing the original list. The reverse() function modifies the original mutable list in place.

• list.reversed(): returns a new reversed list
• mutableList.reverse(): reverses the list in place

kotlin

val list = listOf(1, 2, 3, 4)
val reversedList = list.reversed()

val mutableList = mutableListOf(1, 2, 3, 4)
mutableList.reverse()

Example

This example shows how to reverse an immutable list using reversed() and how to reverse a mutable list in place using reverse().

kotlin

fun main() {
    val list = listOf("a", "b", "c", "d")
    val reversedList = list.reversed()
    println("Original list: $list")
    println("Reversed list (new list): $reversedList")

    val mutableList = mutableListOf("a", "b", "c", "d")
    mutableList.reverse()
    println("Mutable list after reverse(): $mutableList")
}

Output

Original list: [a, b, c, d]
Reversed list (new list): [d, c, b, a]
Mutable list after reverse(): [d, c, b, a]

Common Pitfalls

A common mistake is expecting reversed() to change the original list. It does not; it returns a new reversed list. Also, reverse() only works on mutable lists and modifies the list in place, so it returns Unit (no value).

kotlin

fun main() {
    val list = listOf(1, 2, 3)
    list.reversed() // returns new list but original list stays the same
    println(list) // prints [1, 2, 3]

    val mutableList = mutableListOf(1, 2, 3)
    val result = mutableList.reverse() // reverse() returns Unit
    println(result) // prints kotlin.Unit
    println(mutableList) // prints [3, 2, 1]
}

Output

[1, 2, 3]
kotlin.Unit
[3, 2, 1]

Quick Reference

Function	Description	Modifies Original List?
reversed()	Returns a new list with elements reversed	No
reverse()	Reverses a mutable list in place	Yes

Key Takeaways

Use reversed() to get a new reversed list without changing the original.

Use reverse() to reverse a mutable list in place.

reverse() returns no value; it modifies the list directly.

Immutable lists cannot be reversed in place; use reversed() instead.

Remember that reversed() does not change the original list.